Perhaps the biggest star of that next wave so far is WhatsApp. The messaging service’s founders said last month they had sold for $19 billion to Facebook Inc. Mr. Ballmer questioned whether the company would prove worth the price.

“Is it a fad?” he said. “Well, probably not. It looks more like text [messaging.] I don’t know whether they’ll be successful or not.”

“Will that asset ever be worth anything? Will those 450 million people ever generate enough revenue?” he said. “Reasonable people – Zuckerberg believes so, and no reason to doubt it.”

Mr. Ballmer was speaking Tuesday at Oxford University, at his first public appearance after handing over the CEO role last month to Satya Nadella.

“I’m a very interested board member,” he said of the company he joined in 1980 and headed since 2000.

“I own 4% of Microsoft,” he said. “I care a lot about my child, and my investment, and therefore the investment of the other owners of our company.”

Mr. Ballmer gave advice and answered questions for an hour to a packed room of students in a talk punctuated with plenty of laughs and hand clapping from the 57-year-old Detroit native. He told students that the toughest decisions he has had to make were hiring – and firing – the right people.

His successor has been doing just that. Mr. Nadella said Monday that several top executives would be replaced in the transition to new leadership. Mr. Ballmer didn’t specifically address that, and he said Microsoft has a “great team who will speak on behalf of the company.”

Mr. Ballmer said he sees technology playing a major role in two areas in the future: bringing the next billion people into the consumer economy, and disrupting entire fields that are still slow to change, healthcare and education.

Mr. Ballmer said that for him, the long-term goal would be to make more money for shareholders than they expect.

One student asked what it was like to be incredibly wealthy and powerful. Mr. Ballmer said that he usually would have punted on a question like that. But now that he’s retired? He felt free to say what he really felt: “I can play just about any golf course I want.”